This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No.P2001-364379, filed on Nov. 29, 2001; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a switching power supply having a configuration in which a DCxe2x80x94DC converter is connected to an output side of a power factor improving converter, in particular, to a switching power supply capable of reducing its power consumption.
2. Description of the Related Art
Conventionally, switching power supplies are known, in which an alternating current power (AC) is converted to a direct current power (DC) by switching the operation and the obtained DC power is output to a load.
FIG. 1 is a circuit diagram showing a configuration of such a switching power supply according to a related art.
The switching power supply comprises a rectifier 2, a power factor improving converter 26 connected to the output side of the rectifier 2, a DCxe2x80x94DC converter 27 connected to the output side of the converter 26, and a controller 15. This controller 15, forming a part of both the converters 26 and 27, controls the operation of the power factor improving converter 26 and the DCxe2x80x94DC converter 27.
The rectifier 2 rectifies an AC voltage from an AC power source 1 and outputs the rectified voltage is supplied to the power factor improving converter 26. The converter 26 improves the power factor of the AC voltage including a ripple current and converts the rectified voltage to a DC voltage which is higher than the rectified AC voltage. The DCxe2x80x94DC converter 27 converts the DC voltage from the converter 26 to the converted DC voltage as the DC power for the load.
The controller 15 comprises a DC/DC controller 12 and a power factor controller (PFC) 13. The DC/DC controller 12 controls the operation of the DCxe2x80x94DC converter 27 and the PFC controller 13 controls the operation of the power factor improving converter 26.
Next, a description will be given of the operation of the switching power supply having the configuration described above.
The voltage rectified by the rectifier 2 is supplied to the DCxe2x80x94DC converter 27 through the power factor improving converter 26. When the voltage is supplied to the DCxe2x80x94DC converter 27, the voltage also supplied to both the DC/DC controller 12 and the PFC controller 13. This supply of the voltage starts up both the DC/DC controller 12 and the PFC controller 13.
The PFC controller 13 performs switching (ON/OFF) control for a first switching element 4 made up of a metal oxide semiconductor (MOS) transistor with a predetermined frequency in order to convert the voltage from the rectifier 2 to a DC voltage by increasing its voltage level.
That is, during ON period of the first switching element 4, a current flows through a reactor 3a, the energy stored in the reactor 3a during the OFF period of the first switching element 4 is supplied to and then charged in a smoothing capacitor 6 through a diode 5.
At this time, the PFC controller 13 performs the ON/OFF switching the operation of the first switching element 4 so that the current flowing through the first switching element 4 becomes the same phase of a sine wave of the AC voltage and the voltages between both terminals of the smoothing capacitor 6 becomes a constant value.
On the other hand, the DC/DC controller 12 performs the switching of a second switching element 8 made up of a MOS transistor in the DCxe2x80x94DC converter 27, for example, with a predetermined frequency, and supplies the power from a secondary winding 9b in a transformer 9 to a load 20. At this time, a voltage is induced in a thirdly winding 9c and the induced voltage is then smoothed by a diode 10 and a capacitor 11.
The smoothed voltage is supplied to both the DC/DC controller 12 and the PFC controller 13 as the power for the controller 15.
However, the power factor improving converter 26 connected to the DCxe2x80x94DC converter 27 in the switching power supply of a related art operates at light load for which no operation of the power factor improving converter is necessary. Accordingly, the power consumption of the switching power supply becomes increased in the light load state. Thus, the switching power supply of the related art involves a drawback that it is difficult to increase its power efficiency.
Accordingly, an object of the present invention is, with due consideration to the drawbacks of the technique of a related art, to provide a switching power supply capable of improving its power efficiency.
According to an embodiment, a switching power supply has a power factor improving converter, a DCxe2x80x94DC converter, and a control section. The power factor improving converter has a first switching element converting an alternating current (AC) voltage to a direct current (DC) voltage which is greater in voltage level than that of the AC voltage by performing a ON/OFF control for the first switching element. The DCxe2x80x94DC converter has a second switching element converting the DC voltage from the power factor improving converter to a DC voltage by performing a ON/OFF control for the second switching element. The control section judges a load state according to a pulse signal to be used for performing the ON/OFF control for the second switching element, and stops the operation of the power factor improving converter when the judgment result indicates a light load state, and starting the operation of the power factor improving converter when the judgment result indicates a heavy load state which is rather in load than the light load state.
According to another embodiment, the control section in the switching power supply has a load state judgment circuit and a power factor improving converter (PFC) ON/OFF switching circuit. The load state judgment circuit judges a load state according to the pulse signal to be used for performing the ON/OFF control for the second switching element. The power factor improving converter (PFC) ON/OFF switching circuit stops the operation of the power factor improving converter when the judgment result indicates a light load state, and starts the operation of the power factor improving converter when the judgment result indicates a heavy load state rather than the light load state.
In addition, according to another embodiment, the load state judgment circuit in the switching power supply has a reference period generation circuit and an ON period comparison circuit. The reference period generation circuit generates a first pulse signal of a first reference ON period according to the light load state and a second pulse signal of a second reference ON period, that is shorter in time than the first reference ON period of the first pulse signal, according to the heavy load state. The ON period comparison circuit compares one of the first reference ON period and the second reference ON period with an ON period of the pulse signal to be used for performing the ON/OFF operation of the second switching element, and switches the output of the reference period generation circuit from the second pulse signal of the second reference ON period to the first pulse signal of the first reference ON period when the judgment result indicates the current state of the load is the light load state.
Further, according to another embodiment, in the switching power supply, the ON period comparison circuit switches the output of the reference period generation circuit from the first pulse signal of the first reference ON period to the second pulse signal of the second reference ON period when the judgment result indicates the current state of the load is the heavy load state.
Moreover, according to another embodiment, the reference period generation circuit in the switching power supply has a hysteresis characteristic in which the reference period generation circuit outputs the first pulse signal of the first reference ON period when the ON period of the pulse signal to be used for switching the second switching element is shorter in time than the second reference ON period, and outputs the second pulse signal of the second reference ON period when the ON period of the pulse signal to be used for switching the second switching element is longer in time than the first reference ON period.
Still furthermore, according to another embodiment, the PFC ON/OFF switching circuit in the switching power supply stops the operation of the power factor improving converter during the light load state in order to reduce the magnitude of the current flowing through the power factor improving converter.
In addition, according to another embodiment, the load state judgment circuit in the switching power supply determines that the current state is the light load state when a power consumption of the load is not more than a high frequency wave control target power.
Moreover, according to another embodiment, the control section in the switching power supply controls that the ON period or the pulse signal to be used for switching the second switching element is decreased according to decreasing the load while keeping the output voltage of the switching power supply to a constant level.
The above and other features and advantages of this invention and the manner of realizing them will become more apparent, and the invention itself will best be understood, from a study of the following description and attached claims, with reference had to the attached drawings showing some preferable embodiments of the invention.